They didn't have PC when I first heard that joke.
Well, then maybe you just needed a hearing aid. That aint the way the joke goes.
Actually, that's not even the whole joke. The real punchline comes at the end, when the Lone Ranger says:
"Look, Injun, I'm gunna steal a line from Willie Shakespeare to tell you before I smoke your sorry ass: Et tu, Redskin?"
Then he busts a cap in Tonto's sorry, back-stabbin, heathen ass, which is the funny part.